Various types of dispersion machines have been developed as the above-mentioned machines for performing dispersion or grinding processing. Among such dispersion machines, there is a colloid mill-type dispersion machine.
This dispersion machine includes a pair of upper and lower disk-shaped grindstones, and the upper and lower grindstones are relatively rotated with their axes aligning with each other. The granular material (material to be processed) that is supplied to a central charging part is thereby atomized in the course of being discharged to the outer periphery through a gap between the grindstones (for example, refer to Japanese Unexamined Patent Publication No. 2000-153167).
Meanwhile, with the dispersion machine of Japanese Unexamined Patent Publication No. 2000-153167, since the peripheral velocity at a portion near the axis of the grindstone is different from the peripheral velocity at a portion near the periphery in the gap between the grindstones, the shearing force applied to the material to be processed is smaller at the portion near the axis than at the portion near the periphery. Accordingly, since the material to be processed moves in a shearing force distribution having a gradient of shearing force, a difference in the shearing force that is applied to the material to be processed will arise depending on positions where the material to be processed moves, which causes a problem that variations tend to arise in the dispersion processing.
Moreover, with the dispersion machine of Japanese Unexamined Patent Publication No. 2000-153167, since there is a considerably great gradient in the shearing force distribution in the gap (dispersion region) between the upper and lower grindstones, it is difficult to apply a relatively stable shearing force to the material to be processed. In particular, there is a problem that a sufficient shearing force cannot be applied at a portion near the axis of the grindstones in the gap. In addition, with the dispersion machine of, a lower surface of the upper grindstone and an upper surface of the lower grindstone are not flat and are formed at a predetermined inclination. Thus, since the gap between both grindstones will change in the circumferential direction and the radial direction, the material to be processed in the form of a fluid existing in the gap will be seen to have changed viscosities in view of Newton's well-known viscosity equation, which causes a problem that dispersion cannot be performed efficiently.
The dispersion machine of Japanese Unexamined Patent Publication No. 2000-153167 will encounter the same situation when used for grinding a solid.